1. Field of the Invention
The present invention relates to measurement of a density of a particulate matter (such as soot made of carbon particles, high molecular hydrocarbon particles, and sulfur particles such as sulfate) in a collector of the particulate matter using an electromagnetic wave (frequency thereof is equal to or more than 0.01 [THz], and equal to or less than 100 [THz]) (such as a terahertz wave (frequency thereof is equal to or more than 0.03 [THz], and equal to or less than 10 [THz]), for example).
2. Description of the Prior Art
It has conventionally been known to measure the quantity or density of a particulate matter in a particulate collection filter referred to as a diesel particulate filter (DPF) for a diesel engine (or for particulate collection and combustion catalyst for a diesel engine) using an electromagnetic wave including a terahertz wave or micro wave (refer to Patent Documents 1, 2 and 3).
According to the above-mentioned conventional technologies, the intensity of the electromagnetic wave which has transmitted through the DPF (or combustion catalyst) is measured, the absorption coefficient α′ of the electromagnetic wave is derived, and the density (or quantity) of the particulate matter is measured. Moreover, the absorption coefficient α0′ of the electromagnetic wave when the density of the particulate matter is zero, and an increase rate β′ of the absorption coefficient of the electromagnetic wave with respect to the density of the particulate matter are measured in advance. The density of the particulate matter is represented by (α′−α0′)/β′, and the density of the particulate matter can thus be obtained.
The DPF itself absorbs the electromagnetic wave, and α0′>0 thus holds. Therefore, it is conceived that the absorption coefficient of the electromagnetic wave of the DPF when it is new (without adhered particulate matter, of course) is measured, and the measurement value is set to α0′.    (Patent Document 1) Japanese Laid-Open Patent Publication (Kokai) No. H6-288220    (Patent Document 2) Japanese Laid-Open Patent Publication (Kokai) No. 2009-2276    (Patent Document 3) Japanese Laid-Open Patent Publication (Kokai) No. 2009-57948